1. Field of Invention
This invention relates to a method of preparing certain tetra-aryl-polymethine compounds. These molecules are useful as dyes and sensitizers for thermographic, photographic and photothermographic recording materials.
2. Background of the Art
Polymethine dyes of formula I are well known in the art as having utility in recording materials responsive to a wide range of irradiative energy wavelengths, from thermal exposure to infrared to visible light. U.S. Pat. No. 5,135,842 describes the use of polymethine dyes in thermal dye bleach constructions for imaging and for antihalation layers. U.S. Pat. No. 4,939,117 describes the preparation of pentamethine dyes and their use as leuco dyes. U.S. Pat. No. 4,948,715 and European Patent Application No. 0,353,007 describe the preparation of pentamethine dyes and their use in optical recording.
In polymethine dyes of formula I, typically, A.sup.1 -A.sup.4 and n are chosen so as to be responsive to particular wavelengths as appropriate for the recording to be done, and include amino groups, alkyl- and aryl-substituted amino groups, alkoxide groups, and other substituents which may advantageously contribute or donate electrons to the polymethine system. Usually, A.sup.1 and A.sup.2 are identical to each other, and A.sup.3 and A.sup.4 are identical to each other, and often, due to the method of synthesis of the polymethine system, all four aromatic substituents A.sup.1 -A.sup.4 are identical. Typically, n is 1, 2, or 3. The counterion X.sup.- can be any of a large number of appropriate anions derived from organic or inorganic acids. ##STR1##
Mixtures of dyes exhibit mixed absorption spectra, which may not be useful in applications requiring absorption at a single, specific wavelength. Purification of mixtures of polymethine dyes obtained from strong acid-catalyzed condensation reactions has proven to be very difficult, relying on selective fractional crystallization methods, which give poor yields, inadequate separations, and often are not reproducible on a production scale. Pure polymethine dyes are needed in the color imaging art for true color reproduction. For example, when used in thermally bleachable antihalation layers of color imaging constructions, impure dyes cause undesirable color to remain after bleaching.
The two well-known methods of synthesis of the pentamethine dye system are described in U.S. Pat. Nos. 4,939,117 and 4,948,715. U.S. Pat. No. 4,939,117 describes a method whereby 2 equivalents of a 1,1-diaryl-ethene are coupled with 1-equivalent of triethylorthoformate in an acid-catalyzed condensation in which water is eliminated and a pentamethine dye salt of formula I is obtained. In this synthesis, substituents A.sup.1 and A.sup.2 will be, by necessity, identical to A.sup.3 and A.sup.4 and no possibility exists for formation of a polymethine dye having substituents with A.sup.1 and A.sup.2 different from A.sup.3 and A.sup.4. U.S. Pat. No. 4,948,715 describes a method whereby a 1,1-diaryl-ethene is coupled with a 3,3-diaryl-2-propenal in the presence of a strong acid which simultaneously effects condensation and elimination of one water molecule to form a pentamethine dye salt of formula I. In this method, a possibility exists to obtain an tetra-aryl polymethine dye whereby A.sup.1 -A.sup.4 are different since the substituents on the 1,1-diaryl-ethene could easily be different from those on the 3,3-diaryl-2-propenal. However, mixtures of pentamethine dyes usually result from this method, possibly due to an acid-catalyzed transformylation and equilibration of the aldehyde moiety between the two diaryl-ethenes. The result of this second synthetic method is usually an undesirable mixture of the desired polymethine dye with "crossed-reaction" products having either A.sup.1 and A.sup.2 groups or A.sup.3 and A.sup.4 groups on both ends of the polymethine chain.
European Pat. Application 0,353,007 describes the synthesis of pentamethine dyes of formula I in which one of the four A groups is different from the other three identical A groups. These dyes were prepared from the acid-catalyzed condensation of a 1,1-diaryl-ethene in which the two aryl groups in the 1-position were different with a 3,3-diaryl-2-propenal in which both aryl substituents of the aldehyde were identical to one of the aryl substituents of the 1,1-diaryl-ethene.
Thus, although the art provides means of preparing 3,3-diaryl-2-propenal compounds and 1,1-diaryl-ethene compounds, no general means of condensing these compounds in a manner which will ensure the absence of "crossed-reaction" polymethine condensation products has been disclosed. Thus, there is a need for a method of producing tetra-aryl polymethine dyes in good yield which allows preparation of materials without contamination from side reactions that result in the formation of undesired "crossed-reaction" products. Such a method has now been discovered.